I want to sketch out Thomas Aquinas’ theory of natural law by distinguishing between the four kinds of law he outlines in the Summa Theologiae and then discussing his conception of the Good. Afterward, we’ll ask if Aquinas’ view is compatible with a biblical view.
Thomas Aquinas synthesizes philosophy and theology to arrive at his theory of natural law. In the Summa, he begins his treatise on the essence of law by stating the definition of law in general as “a dictate of practical reason in the prince who governs some perfect community.” Thus, law has an essential relation to reason. By distinguishing between four kinds of law, Aquinas demonstrates that ultimately, all law is subject to Divine Reason, or eternal law.
#1—Eternal Law: This first law represents the timeless principles found in the physical and moral world by which “the whole community of the universe is governed.” Since “all laws proceed from the eternal law,” ontologically, the eternal law is the measure of all other laws, and thus they find their derivation from the eternal law.
#2—Natural Law: Aquinas begins his discussion of the second kind of law, natural law, by making a distinction. Not only is a law in the reason of a ruler, but it “can be in something...as in the ruled and measured.” More precisely, natural law is that law which is imprinted upon our human nature and directs us toward our natural good or end. In reference to Aquinas’ view of natural law, J. Budziszewski terms this kind of law “as moral principles that we cannot not know” (from Written on the Heart, p. 61). As such, natural law is a reflection of the eternal law found within the structure of human reason.
#3—Divine Law: The third law, divine law, is defined as that law which is revealed in the Christian Scriptures. In contrast to the teleology of natural law, divine law pertains to our “supernatural ultimate end”: our reconciliation with God Himself. According to Aquinas, there are two kinds of divine law. There is the old law, found in the Old Testament, and the new law, found in the New Testament. Divine law is necessary because unaided human reason cannot apprehend the whole of eternal law. Thus, divine law is also a reflection of the eternal law in the form of special revelation.
#4—Human Law: Lastly, Aquinas sets out a fourth law, human law. Human law is simply the derivation of civil law (or the laws of a nation) from the principles of natural law. In this kind of law we observe that “human reason must proceed to dispose of more particular matters.” Thus, human law in the United States would include traffic laws, tax laws, specific criminal laws, etc.
With a description of Aquinas’ four kinds of law, the necessary framework is in place to understand his conception of the Good and its relation to his natural law theory. Following Aristotle, Aquinas held that to understand the essential nature, or essence, of a thing (and thus, the Good of that thing), one must know its telos (from the Greek meaning purpose or end). For Aquinas, the end of man is human flourishing. However, in contrast to Aristotle, Aquinas’ theological underpinnings provide a more robust view of human flourishing, one that is in accordance with the Design Plan or eternal law of God. Therefore, the Good of man is the perfecting of a human nature that reflects the eternal law, which is accomplished through the perfecting of human flourishing in accordance with the eternal law.
Aquinas clarifies his view of human nature by outlining the four basic human goods we discover by observing our natural inclinations. Those goods are life, procreation, knowledge and sociability. The realization of these basic ends constitutes the perfection of human nature (the Good). With this Good in mind, it follows that the rules for right conduct would be conducive to human flourishing if they are derivative from laws that are natural to us. Therefore, Aquinas’ theory of natural law enables us to share in God’s eternal law by providing basic moral principles grounded in human nature and “that we cannot not know,” from which we derive human laws (or rules of right conduct) that are completed by the revelation of divine law found in the Christian Scriptures.
So is Aquinas’ account compatible with biblical Christianity? Well clearly God’s eternal law and His divine law and human law, as Aquinas defines them, are seen and referenced in the biblical record. But what about natural law? I think so and here’s why.
Romans 1:19–20 indicates that God provides general revelation to all, such that man can see and understand His “eternal power and divine nature” in the created order. How is this so? According to Genesis 1:26–27, man is made in the image of God, and thus we are created with the rational and moral capacities that enable us to see the truth of God in general revelation. Furthermore, Romans 2:14–15 states that men “instinctively know what the law requires” because “what the law requires is written on their hearts.” Thus, as creatures with rational faculties we are able to discern God’s natural law through His creation and our own human nature.
At this point, one might object: If the preceding account is true, doesn’t it follow that all men should know right and wrong? However, all men do not know right and wrong, and even when they are able to differentiate, they do not always choose what is right.
A further examination of the biblical record not only states this reality (Romans 7), but it gives us an adequate explanation for the state we find man in. According to Romans 1:18, men “by their wickedness suppress the truth.” In other words, man is sinful and this condition leads to his denial of the truth of natural law. As Budziszewski argues, “we pretend to ourselves that we do not know what we really do know” (from Written on the Heart, p. 182). In addition, as Romans 6:23 states, the penalty of our sin and suppression of the truth is death.
Fortunately, in His unspeakable grace and mercy, God has not only revealed Himself in the created order but He has revealed Himself in the person of Jesus Christ, so that He might remedy our hopeless condition and provide a way in which man might fulfill his ultimate end: reconciliation to God Himself. As Romans 5:11 promises, “through our Lord Jesus Christ...we have now received reconciliation.” Amen.
*Aquinas quotes taken from the Summa Theologiae, ed. and trans. by Ralph McInerny (London: Penguin Books, 1998).